The trend toward increasing the number of functions of an integrated circuit device (IC device) is continuing. As the size of transistors decreases, serious drawbacks in current transistor fabrication processes become evident. For example, typical silicon-on-insulator (SOI) transistors are fabricated by coating a substrate with an insulator (e.g., glass or silicon oxide) layer. A second silicon wafer is then bonded to the insulator layer and thinned to a desired thickness (i.e., as determined by the transistor dimensions). This thinning process is very difficult to control with great accuracy.
FIGS. 1A-1D illustrate a portion of the fabrication process for creating a tri-gate SOI transistor in accordance with the prior art. As shown in FIG. 1A, a carrier wafer 101, typically a silicon substrate, has an insulator layer 102, typically silicon dioxide, disposed upon it. For example, a silicon dioxide layer may be grown on a silicon substrate.
As shown in FIG. 1B, a transfer wafer 103 is then bonded to the insulator layer 102, which may facilitate the bonding. The bonding of the carrier wafer to the insulator layer may be effected though a heat-induced hydrogen bonding process. The transfer wafer, which may be, for example, silicon, is approximately 600 microns thick.
The transfer wafer is then thinned to a desired thickness based upon the transistor dimensions. Typically, this thickness is approximately 50-100 nm. The thinning of the transfer wafer may be accomplished through one of several typical processes. For example, a wet etch and polish process may be used to grind the transfer wafer to the desired thickness. An alternative method for thinning the transfer wafer includes hydrogen implantation of the transfer layer to create a weak section of the transfer wafer. The bonded pair is then heated to effect a high temperature cleave of the hydrogen-doped interface. Subsequently, the transfer wafer surface is polished or treated in other ways to planarize the surface or further reduce the thickness. These methods provide control of the thickness to within approximately several hundred angstroms. As shown in FIG. 1C, the transfer wafer 103 has been thinned to a desired dimension for the silicon body of the transistor, resulting in film layer 104. The thickness of film layer 104 is determined by the desired height of the silicon body (HSi). The film layer 104 is then selectively etched to create silicon bodies for the transistors. As shown in FIG. 1D, selectively etching the film layer 104, using lithography techniques, results in silicon bodies 105 having a desired body width (WSi) and body height (HSi).
For typical transistor design architecture, gate length is proportional to HSi, with HSi equal to about one-third of gate length. For typical transistors with gate lengths of approximately 20-100 nm, the desired HSi is greater than approximately 20 nm. Using the current fabrication method, it is possible to create adequate film layers. However, as the gate length, and hence, the desired HSi decreases, current fabrication methods exhibit serious disadvantages.
The HSi value must be uniform across a wafer in order to produce transistors with uniform characteristics. For example, the transistor threshold voltage, which is directly proportional to HSi, should not vary by more than approximately 10%. Therefore, the film layer thickness that determines HSi, should not vary by more than 10%.
The methods of thinning the transfer layer to obtain the film layer are capable of producing a film layer of approximately 20 nm thickness that does not vary by more than approximately 10%. However, these methods fail to produce the required uniformity for thinner film layers. Therefore, current methods of fabricating SOI transistors are incapable of yielding transistors with gate lengths smaller than approximately 50 nm.
Moreover, the process of bonding the carrier wafer and transfer wafer, and the process of thinning the transfer wafer to the desired thickness, are costly and difficult to control.